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Myths and Opportunities for Ink & Coatings Adhesion

Steven Abbott – Sep 20, 2019

TAGS:  Science-based Formulation      Adhesion Promoters      Adhesion Basics    

Myths and Opportunities for Ink & Coatings Adhesion The last thing an ink and coatings formulator needs is the wrong scientific advice. Formulation is hard and it is even harder if time and energy are spent on the wrong things.

In adhesion, a lot of time is spent worrying about wetting, contact angles & surface energies, and it turns out that most of this is a waste of precious resource. Here, I want to free up your resources to concentrate on the things that really matter for robust adhesion.

Let’s start with all surface energy measurements before focussing on an effective approach to obtain a strong adhesion...

Adhesion & Coatability Don't Depend on Surface Energy

If the surface energy of your surface goes from 32 dynes/cm to 42 dynes/cm & adhesion depends on surface energy, then you have increased adhesion by 42/32, just a 30% improvement, insignificant compared to what is needed.

If we measured adhesion via peel, the surface energy would go from 32 mN/m to 42 mN/m. And yet a modest real-world adhesion is 100 N/m, more than 2000x larger than surface energy. So in terms of adhesion, surface energy is a waste of time – adhesion is created by a different set of mechanisms.

But what about applying the ink or coating; surely the wetting behavior is important? For inkjet this is true. A drop of ink flies through the air and meets a surface and the contact angles very much control things. But for everything else, the wetting of the surface is 100% irrespective of the surface.

An offset or flexo-plate forces the ink into contact and a slot or roll coater does the same. So, the difference between an easy and difficult surface is not “wetting” but “dewetting”, and the timescales of interest are not the leisurely few seconds of a typical contact angle experiment but milliseconds before drying commences.

Very few people measure dewetting because it is difficult, and it is even more difficult to do so over the relevant timescales. If you go to my dewetting app and put in a typical printed dot of 100µm radius and 5µm height, even if the static contact angle is an acceptable 40°, the dot will have dewetted in a few milliseconds, making printing impossible.
Dewetting App
Fig 1. The dewetting app shows that a typical printed dot will dewet in ~20ms, even though its contact angle would be seen as “normal”.

We hear a lot about wetting and very little about the more-important (and more difficult to study) dewetting. This app shows that a typical printed dot will dewet in ~20ms, even though its contact angle would be seen as “normal”.

Understanding why our inks don’t dewet this quickly is far more important than worrying about wetting.

The fact that printing is possible shows that something is inhibiting dewetting. This must mean that the ink is affecting the surface in such a way that as the ink starts to dewet there is a small surface component (such as a surfactant) that encourages a contact angle of 0°, which means local wetting, which means no dewetting.

So, the difference between a good ink and a bad ink is not one to do with standard surface energies, but about high-speed surface interactions with one or more of the ink ingredients.

For most of us, there is no need to worry about this because dewetting of most inks/coatings on most surfaces is not an issue. For those who have dewetting issues, finding specific additives to discourage dewetting rather than worrying about meaningless advancing static contact angles is a much more productive use of time.

Three-ways to Get Strong Adhesion

In other places, I emphasize that Adhesion is a Property of the System, because it turns out that there are many ways to increase adhesion without doing anything to the key adhesive itself. But here, we want to focus on the three things that make a fundamental difference to adhesion across an interface.

1. A Clean Surface

The first is obvious – that surface must be clean and free of things like oil residues that can destroy adhesion. One way to achieve this is by roughening the surface. It is a common myth that roughening increases the surface area that automatically gives greater adhesion. This myth arises when people look at a typical scan of surface roughness.

Surface Profile Explorer scan
Surface Profile Explore
Fig 2. Surface Profile Explorer app

Figure 2 describes a typical scan of a surface (top) seems to show that it’s super rough, with lots of extra surface area. But, when you use the Surface Profile Explorer app and expand the X-axis you find (bottom) that it’s nearly flat.

You can use an app to explore surface roughness in detail, but here I want to focus on one measure that can be obtained by most good surface measuring devices, Lr, the Length Ratio. Lr is the distance travelled if you follow the contours of the surface up and down, divided by the distance if the surface was perfectly flat. For most practical surfaces to which we want to stick things (and in the specific example shown in Fig 1), Lr is close to 1.000, i.e. to 3 decimal places there is no extra surface that can give extra adhesion. We are deceived by the scale. In the X-direction we have 12500µm and in the Y-direction we have just a few µm.

When we magnify the X-direction to be more comparable (the right of Fig 1), we see that the surface is very gentle, creating almost no extra surface and not providing any potential for “mechanical interlocking”, another common adhesion myth.

Why, then, does roughening the surface increase adhesion? I have scanned 100s of papers on the topic & the answer is that often it has no significant effect. Sometimes it makes things worse, and when it does work, it’s probably because the roughening has removed surface junk.

Substrate Surface Analysis: Get the Most from Roughness Data

2. Polymer-polymer Entanglement

Now, let’s switch to a positive message about good adhesion. Here the trick is simple:

  • Make sure that the polymers in your ink or coating are in a reasonable solvent which is also compatible with the polymer surface.
  • The solvent will help the polymers to entangle across the interface and this entanglement gives strong adhesion.

The reason is that, if you try to pull the interface apart, the energy is dissipated across a wide, entangled area rather than focussed along the interface. Rigid interfaces cannot dissipate energy, so a crack can readily propagate.
Entanglement of Polymer
Fig 3. The polymer chains are entangled, like spaghetti. If you try to pull them apart they spread the load over a large distance, dissipating the energy trying to force the surfaces apart.

If you have a polymer surface such as polyethylene (PE), a solvent like xylene should do a good job in allowing entanglement with, for example, a rubber-based coating. As we all know, this doesn’t happen, the reason is that the PE is inert to most solvents because of its crystallinity. Similarly, a ketone solvent should be great for sticking an acrylate formulation to PET, but PET is inert to most solvents.

Crystalline PE Entanglement
Fig 4. The crystalline PE surface not allowing any entanglement (L). On treatment with corona, the surface can open up, allowing easy entanglement with a similar polymer and an appropriate solvent (R), thus creating extra functionality.

In both cases, the crystallinity needs to be reduced. For PE and PET, this can be done via a corona, plasma or flame treatment. A side effect of such treatments is that the surface energy increases, but because surface energy is irrelevant to strong adhesion, this effect is of no consequence.

An interesting proof of this comes from a less-well-known surface treatment of PET.

  • Hit it with a xenon flash (now popular for photonic-curing) and adhesion to PET via, say, ketone solvents becomes easy.
  • The flash has heated the top few nanometers to well above PET’s melting point, then the surface has cooled super-quickly, leaving the PET in an amorphous state which is easy to stick to.
  • There is no change to surface energy & surface functionality, but a large increase in adhesion via standard solvent-polymer-polymer interactions.

How do you ensure that polymers & solvents are compatible, and how do you control the solvent blend to ensure that you don’t destroy the surface of, say, polycarbonate? Regular users of SpecialChem will know of Science-Based Formulation using Hansen Solubility Parameters to control these interactions via the idea of HSP Distance, where a small distance means compatibility and a large distance means incompatibility. Learn the XL Power of HSP for Coatings Compatibility Issues Today!

3. Chemical Bonds Across the Interface

It seems obvious that to get strong adhesion across an interface, we should line up lots of chemical bonds that go from one surface to another. Interestingly, this is a way to get poor adhesion. This is very good news because, in practice, we would find it hard to create lots of chemical bonds across the interface, during most printing/coating processes.

The problem with lots of chemical bonds is that they give a brittle interface that is easily cracked, just like glass. What we really need are a few bonds that allow the whole interface to be entangled.
APTES on aluminium surface
Fig 5. Reacted APTES onto an aluminum surface, creating the strong aluminosilicate bonds, leaving the amine groups able to react into another system such as epoxy or urethane.

If we are trying to stick to aluminum, we can add something like APTES (aminopropyl triethoxysilane), which is a bi-functional molecule. APTES has a triethoxysilane at one end which loves to attach to the aluminum (via its oxide surface), and an amine group at the other that loves to react to epoxies, urethanes or acrylates.

Many times, it has been observed that increasing the level of APTES from 0 to 1% increases the adhesion, but going from 1% to, say, 1.5% makes things worse. This is because we go from a nicely entangled adhesion to a brittle, over-reacted interface.

A beautiful example of obtaining strong adhesion via low levels of chemical bonds is when we want to stick PE to something like an epoxy where we know (via HSP) that there is no chance of polymer-to-polymer direct entanglement because the systems are too incompatible.

Instead, we do a light corona treatment that creates a low level of carbonyl and carboxyl functionality, as in Fig 4. It also creates some -OH functionality which increases surface energy but is useless for the real adhesion process.

What happens next is that we coat a thin layer (25-50nm) of polyethyleneimine (PEI), which contains primary amine groups which react with some of the carbonyl and carboxyl groups, leaving plenty of free amines for the final step. When an (epoxy or urethane or acrylate) ink or coating is applied, free-amines react into the coating, forming an entangled network from the PE to the coating, and giving a strong adhesion.

PEI Entanglement
Fig 6. The carbonyl and carboxyl on the PE surface have first reacted with primary amines on PEI, then more of those amines have been reacted into an epoxy or urethane matrix, creating a strong entangled system.

The PEI is a very weak polymer and is usually provided with extra crosslinks to reduce its weakness. Even a thick layer (100nm) of PEI is bad for adhesion because the system can fail cohesively within the PEI. By having a layer as thin as possible, the weakness of PEI is less important and, as a bonus, there is less tendency to attract water to the interface which would harm long-term stability.

Science-based Adhesion

Once you get into the habit of thinking about the true science of adhesion, it is liberating. You don’t have to waste time on surface energy or worry about creating lots of chemical bonds. You just focus on whatever will give you strong entanglement across the interface, always remembering that too much of a good thing is a bad thing – you make matters worse by trying too hard.

  • If you know, from Hansen Solubility Parameters, that you have good compatibility across the interface, then you can tune your solvent system to provide just the right amount of “bite” into the surface to get entanglement without destroying the surface.

  • If you have a crystalline surface then you need to open it up with whatever process, reducing the crystallinity without destroying the surface.

  • If you use corona, plasma or flame you focus not on surface energy, but on the crystallinity combined with the integrity of the polymer.

  • If you have a UV-absorbing polymer such as PET, the xenon flash trick gives you maximum amorphous surface with minimum destruction of the polymer.

  • If you know you cannot get physical entanglement, you have to find a rational chemistry that gives a few % of useful functionality to entangle the surfaces via a few chemical bonds.

  • If you are going to stick to a metal-oxide surface using, a triethoxysilane, focus your efforts on providing a stable oxide that reacts well with the silane and which isn’t too sensitive to whatever environment your product will meet during its use. And make sure that the other end of the adhesion promoter reacts quickly and permanently with the other surface.

Above all, don’t try too hard. I’ve had experience of several adhesion crises where I suggested that they decrease the temperature or UV intensity or level of adhesion promoter. They think that I’m mad because more is always better. But sure enough, when out of desperation they tried less hard, adhesion becomes strong.

What should we measure on the surface to better understand our adhesion? The answer is clear because we know the key mechanisms. For example, if we want to stick to PE we need to know:

  • The degree of crystallinity (before and after treatment)
  • The degree of de-polymerization
  • The amount of useful functionality (e.g. carbonyl and carboxyl for amine reactions), and
  • The amount of useless functionality (e.g. -OH if we want to react with amines)

These are difficult challenges. But supply and demand are powerful forces. Because we have demanded surface energies, we have been supplied with surface energy equipment. Once we consistently demand metrics of what really matters, the industry will be able to respond.

We know that XPS and TOF-SIMS can provide some of this information, but often these techniques are too expensive for us. There are plenty of simpler techniques out there – maybe we haven’t tried hard enough to use them to give us the answers we need to formulate more scientifically.

The take-home message is that we get what we ask for. If we switch our requests towards techniques that will help us formulate more scientifically, we will get better adhesion for less work. That’s something worth striving for.

Note: Interested readers can find many more apps covering the science of adhesion on the author’s Practical Adhesion website. All the apps are free and safe to use in a corporate environment. The author’s book, Adhesion Science: Principles and Practice is available from DesTECH Publications.

Maximizing Adhesion in Coatings – Formulations & Processes

Promote and maintain good adhesion and durability of your adhesives, coatings & inks by better understanding adhesion science and how it translates into formulation & processes choices. Get tips by leading industry expert Edward Petrie now.

Maximizing Adhesion in Adhesives & Coatings

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